Plot

A magical mirror is stolen by Demona, who frees the mischievous Puck from it; purposely re-interpreting Demona's wishes, he makes the Gargoyles human and the humans gargoyles. In the chaos, Goliath and the others are able to detain him and send Demona fleeing, but as a gift for the fun he had, Puck grants Demona the ability to become human during the day instead of returning to her vulnerable stone sleep.

Trivia

Puck reveals Demona still has some affection for Goliath.

With the exception of Elisa and the Manhattan Clan (who had help in remembering that they'd been transformed), anyone who is/was changed between being a gargoyle and a human has/had their memories altered to believe that they had always been such.

With Puck switching everyone's species around, Elisa and Goliath are able to realize that they have feelings for each other.

In the episode, Demona gains the power to transform into a human during the day. It's later revealed to be quite painful, due to the changes happening in quick succession.